The present invention relates to a reproducing apparatus of a disc on which digital signals are recorded and more particularly to a disc reproducing apparatus suited to improve the reproducing access speed of an optical memory disc on which digital signals are recorded at a constant linear velocity.
One of optical memory discs on which digital signals are recorded is a compact disc (hereinafter abbreviated to a CD). On a CD, audio signals which are converted to digital data are recorded at a constant linear velocity (hereinafter abbreviated to a CLV) so as to increase the recording density. A CD player for reproducing a CD rotates the disc under the CLV control as disclosed in Japanese Patent Application Laid-Open Sho 59-185071 (a first conventional example). To detect digital data recorded by a non-contact optical pickup, the access speed as an audio output device is fast. Furthermore, a subcode as address information is also recorded on the disc, so that the head of the desired music can be searched easily.
As an optical memory disc on which digital signals are recorded, there is a CD-ROM using a CD as a data memory available beside the above. When the CD-ROM is reproduced, it is required from a part as a data memory that the access speed is fast. Rotational speed control for a disc may be cited as one of the factors for deciding this access speed and the ratio of rotational speed between the inner diameter zone of a disc and the outer diameter zone is about 2.3 under the CLV control, so that the time required for the number of revolutions to reach a constant linear velocity comes into a problem.
For this problem, as disclosed in Japanese Patent Application Laid-Open Hei 5-250804 (a second conventional example) proposed by the applicant of this patent, an art for controlling a disc recorded at a CLV at a constant angular velocity (hereinafter abbreviated to a CAV) for reproduction and improving the access speed for disc reproduction has been proposed.
As disclosed in Japanese Patent Application Laid-Open Hei 6-89506 (a third conventional example) proposed by the applicant of this patent, an art for controlling a disc recorded at a CLV for normal reproduction, controlling the rotational speed so as to keep it unchanged by changing the linear velocity for access from the inner diameter zone of the disc to the outer diameter zone or from the outer diameter zone to the inner diameter zone, and furthermore changing the frequencies of the digital data sampling clock and the clock for data processor and RAM controller of the reproducing digital signal processing including the CLV controller in correspondence with the transmission rate of the digital data picked up from the disc has been proposed.
When a CD-ROM is reproduced under the CLV control described in the aforementioned first conventional example, the ratio of rotational speed between the inner diameter zone of a disc and the outer diameter zone is about 2.3 as described above, so that it takes a lot of time until the number of revolutions reaches a constant linear velocity for access from the inner diameter zone of the disc to the outer diameter zone or from the outer diameter zone to the inner diameter zone and there is a problem imposed in the access speed for the data memory. This access speed depends on the performance of a motor for rotating a disc. However, improvement of the performance of the motor is naturally limited.
In the aforementioned second conventional example, since a motor is controlled at a CAV, a problem arises that it is necessary to provide a means for detecting rotational speed and the constitution of the motor is complicated.
According to the art disclosed in the aforementioned third conventional example, the CAV control is necessary during the period of access. Therefore, a problem arises that the CLV controller is complicated, and a circuit for changing the clock frequency for data processor and RAM controller is required, and furthermore the burden of the system control microcomputer is increased. Furthermore, it is not taken into account to control the write speed and the read speed from the memory (RAM) for deinterleaving at the time of error correction and prevent the RAM from overflowing or underflowing by following the linear velocity of a disc changing at the time of access.
Therefore, an object of the present invention is to provide a disc reproducing apparatus which can reproduce an optical disc recorded at a CLV such as a CD or a CD-ROM at a fast access speed.
Another object of the present invention is to provide a disc reproducing apparatus with a fast access speed which requires no complicated and expensive additional part such as a means for detecting rotational speed and uses an inexpensive motor with ordinary performance.
Still another object of the present invention is to provide a disc reproducing apparatus in which the RAM neither overflows nor underflows at the time of reproduction.
To accomplish the above objects, the basic characteristic of the present invention is that a means of changing the frequencies of the digital data sampling clock in correspondence with the transmission rate of the digital data picked up from a disc and the clock for data processor and RAM controller of the reproducing digital signal processing means including the RAM controller except the CLV controller by following an change in the rotational speed of the disc motor by the CLV controller, that is, a change in the linear velocity at the time of access is provided. By this characteristic of the present invention, a subcode including a time code can be reproduced by a constitution of a brief CLV controller before the linear velocity is settled at the time of access, and the RAM is prevented from overflowing or underflowing, and the pull-in time after end of access is shortened by operating the CLV controller always by a constant high speed clock, and the access speed for reproducing the disc recorded at the CLV can be improved.
The more detailed characteristics of the present invention are as follows: A disc reproducing apparatus for reproducing a disc on which a digital information signal such as an error correction code or a subcode including a time code is recorded in the predetermined recording format at a CLV comprises:
a driver for rotating the disc at the CLV;
a CLV controller for controlling the linear velocity;
a reader for reading the aforementioned digital information signal from the disc;
a digital signal processor for executing the predetermined reproducing digital signal processing such as demodulation and error correction for the digital information signal which is outputted from the reader in correspondence with the aforementioned recording format;
a RAM for rearranging and memorizing data temporarily so as to perform an error correction corresponding to the recording format;
a RAM controller for controlling the RAM;
a first clock generator for generating a reproducing clock corresponding to the transmission rate of the digital information signal;
an oscillator for oscillating at a fixed frequency;
a second clock generator for generating a first master clock which is a reference of operations of the digital signal processor, the CLV controller, and the RAM controller from a clock by the oscillator;
a third clock generator for generating a second master clock which is a reference of operations of the digital signal processor and the RAM controller without using the oscillator;
a switch for switching the master clock which is a reference of operations of the digital signal processor and the RAM controller to one of the first master clock by the second clock generator and the second master clock by the third clock generator;
an access controller for controlling the CLV controller and the switch; and
a fourth controller for controlling the whole disc reproducing system;
wherein the access controller controls the switch and switches the clock so that when the disc reproducing system is in the normal reproducing operation state, the digital signal processor and the RAM controller operate by the clock generated by the second clock generator and when the disc reproducing system is in the access operation state, they operate by the clock generated by the third clock generator and the CLV controller operates always by the clock generated by the second clock generator.
According to the aforementioned characteristics, the access controller controls so that the digital signal processor and the RAM controller operate by the first master clock generated by the second clock generator when the disc reproducing system is in the normal reproducing operation state and they operate by the second master clock generated by the third clock generator when the disc reproducing system is in the access operation state and the CLV controller operates always by the second master clock generated by the second clock generator, so that even when no linear velocity for access is settled in the conventional constitution of the CLV controller, a subcode including a time code can be reproduced, and the RAM is prevented from overflowing or underflowing, and furthermore the CLV controller is operated always by a constant high speed clock, and thus the pull-in time after end of access can be minimized and the access speed for reproducing the disc recorded at the CLV can be improved.
According to the present invention, when a reproducing clock generator following an input signal and an operation clock generator for a data processor and a RAM controller are structured so that at the time of access from the inner diameter zone of a disc to the outer diameter zone or from the outer diameter zone to the inner diameter zone, they generate a voltage for increasing or decreasing the oscillating frequency of the clock simultaneously with start of access so as to control the voltage controlled oscillator and then generate an operation clock following rotation of the disc, that is, a change in the linear velocity, a subcode including a time code can be detected before a linear velocity is settled, and the RAM can be controlled so as to prevent it from overflowing or underflowing, and furthermore since the CLV controller operates always by a constant high speed clock using a crystal, the settling time for making the linear velocity constant can be minimized and the access time can be improved.